The lunar crustal magnetic anomalies were not produced by impact plasmas

Abstract:

The crusts of the Moon, Mercury, Mars, and many meteorites parent bodies are magnetized. Although the magnetizing field is commonly attributed to that of an ancient core dynamo, a longstanding hypothesized alternative is that the remanent magnetization may have been produced by impact plasmas that can transiently induce fields or amplify the background interplanetary magnetic field. However, the dynamics of such impact-plasma fields have not been studied self-consistently in the context of the coupled interaction of the impact plume, solar wind, and the non-uniform electrical resistivity inside the body. To address this gap, we combined hydrocode impact-physics and magnetohydrodynamic (MHD) simulations to study impact field generation and amplification on the Moon. Considering a diversity of different impact and solar wind conditions 3.5-4 Ga ago, we demonstrate that the resulting fields are weaker than previously predicted. Field strengths are limited due to solar wind variability, the formation of a diamagnetic cavity around the moon by the expanding plasma cloud, and the fact that the induced field is dissipated by ohmic dissipation in the crust. As a result, the maximum crustal field is at least 3 orders of magnitude too weak to explain the magnetization. The elimination of impact plasmas as the sole source of lunar magnetism leaves a core dynamo as the only plausible source of the majority of magnetization on the Moon. This result may challenge impact-magnetization scenarios considered for other bodies, such as Mercury.